1. Field of the Invention
The present invention relates to an optical apparatus such as a video camera, a silver-halide camera and an electronic still camera and, more particularly, to an optical apparatus capable of correcting focus deviation which occurs due to an environmental variation, in an optical system (photographing optical system) having a moving lens group which moves along its optical axis during focusing or zooming, for example, a photographing optical system such as a single-focal-length lens or a zoom lens.
2. Description of the Related Art
In recent years, in the field of optical apparatuses such as cameras, photographing optical systems are becoming smaller and smaller, and the image sizes of solid-state image sensors are becoming increasingly small in diameter. Plastic materials are widely used as optical materials which constitute part of such a photographing optical system. This is because the use of the plastic materials facilitates the molding of lenses and enables the production of lenses having arbitrary shapes, and also realizes marked cost-effectiveness compared to glass materials. For this reason, lenses made of plastic materials are also widely used for viewfinder systems, infrared active type autofocus units and the like.
However, as compared with inorganic glass materials, such plastic materials have a physical nature easily susceptible to environmental variations. The plastic material has a large coefficient of linear expansion; for example, PMMA (polymethyl methacrylate) which is a plastic material, has a representative value of 67.9.times.10.sup.-6 /.degree.C., whereas LaK 14 (manufactured by OHARA) which is an inorganic glass has a one-digit smaller representative value of 57.times.10.sup.-7 /.degree.C. Regarding variations in their indices of refraction with respect to temperature variations, the representative values of PMMA are 1.0-1.2.times.10.sup.-4 /.degree.C., while the D-line refractive indices of LaK 14 are two digits smaller at 3.9-4.4.times.10.sup.-6 /.degree.C. As described above, as compared with the inorganic glass material, the plastic material has optical constants (index of refraction, shape, etc.) which exhibit great variations with respect to temperature variations. Accordingly, lenses made of the plastic material (plastic lenses) show great variations in focal length with respect to temperature variations, as compared with lenses made of the inorganic glass material.
The plastic material has a large coefficient of water absorption compared to the inorganic glass material. For this reason, the optical constants of the plastic lens vary to a great extent with respect to not only temperature variation but also humidity variation, as compared with the lenses made of the inorganic glass material.
In general, if such a plastic lens is employed, a number of problems will arise. For example, if an environmental variation, particularly a temperature or humidity variation, takes place, the optical nature of the plastic lens, such as focal length, varies to a great extent compared to a lens made of the inorganic glass material. In particular, in the case of recent optical apparatuses of reduced size which have a compact photographing optical system, a small solid-state image sensor and high-density constituent elements, the plastic lens will increase the amount of deviation of the image forming plane of the optical system from a predetermined image forming plane by the influence of an environmental variation, such as a temperature or humidity variation.